The invention relates to a mould for pressure casting flat metal products of great thickness and considerable length such as steel slabs intended to be transformed into a sheet by rolling.
A casting method which consists in introducing a ladle containing the metal to be cast inside a vessel which is then closed by a cover applied in a leaktight manner to the upper edge of the vessel has been known and used for a long time. The cover of the vessel carries a tube in refractory material, the lower part of which is immersed in the metal filling the ladle and the upper part of which communicates with an opening which passes through the cover of the vessel equipped with means for connecting to a slide spout for casting the metal in a mould.
The assembly consisting of the vessel containing the ladle and equipped with its closing cover can be brought into a casting position beneath a mould comprising a filling spout in its lower part. The spout for filling the mould is caused to coincide and to come into leaktight contact with the device for connecting the cover to the ladle, then compressed air is conveyed inside the vessel so as to cause the metal to rise inside the refractory tube, then inside the mould, until the latter is completely filled.
By regulating the pressure of the gas conveyed in the ladle, the conditions for casting the metal and filling the mould are perfectly controlled, which makes it possible to obtain castings of a very satisfactory and uniform quality.
This pressure-casting method, described, for example, in patents U.S. Pat. No. 3,888,453 and U.S. Pat. No. 3,590,904, may be applied not only to the production of shaped pieces, but also to the casting of semi-manufactured products such as slabs, blooms, billets and tube rounds.
In the case of slabs, that is to say of flat steel products of great length and considerable thickness, it being possible for this thickness to be between, for example, 60 and 400 mm or more, use is made of moulds of very large dimensions which make it possible to cast slabs whose length may be of the order of 10 m or more and whose width may reach 3 m or more.
In certain cases, this casting method may advantageously replace the method for continuous casting of slabs, according to the nature of the grades to be cast, the tonnages to be produced and the size of the products both in terms of their thickness and their width.
The moulds used for the pressure casting of slabs comprise a support and tilting frame mounted so as to pivot about a horizontal axis so that it may be inclined very slightly, relative to the horizontal plane, before commencing a casting operation. This pivoting of the frame makes it possible to connect the spout for filling the mould to the exit opening of the cover of the vessel brought into the casting position beneath the mould and makes it possible to control the flow of steel over the lower spacer.
The mould principally comprises two lateral walls of large dimensions disposed parallel and opposite one another, the inner faces which are lined with graphite blocks of which form the surfaces of the mould which come into contact with the molten metal in order to delimit the two large faces of the slab.
The lateral walls are mounted on the frame so as to be movable in the direction perpendicular to their moulding faces, that is to say in the transverse direction of the mould corresponding to the thickness of the moulded product.
The closing of the other faces of the cavity of the mould, of substantially parallelepipedal form, is provided by spacers inserted between the two lateral walls which are clamped between these spacers during casting and cooling of the metal introduced into the mould.
The width of the spacers in the transverse direction determines the thickness of the flat product being cast.
A first spacer, or lower spacer, is fixed to the upper part of the support and tilting frame, practically over the entire length of this frame corresponding to the maximum length of the slab which can be cast in the mould.
A second spacer, or upper spacer, is held by suspension devices at a certain height and in a parallel position, above the lower spacer The position of the upper spacer determines the width of the slab cast in the mould. This upper spacer comprises a part which is bent at 90.degree. at its front end, that is to say located towards the part of the mould placed above the casting vessel.
The front spacer is fixed to the front of the mould in an arrangement which is perpendicular to the lower and upper spacers, in the vicinity of their front ends. The lower end of the front spacer is disposed slightly in front of the front end of the lower spacer, the spout for filling the mould being located such that the passage orifice communicates with the space remaining between these two spacers. The front spacer is extended upwards so as to form, with the head of the upper spacer oriented at 90.degree. relative to this spacer, a space communicating with the upper part of the mould providing filling of the header at the end of casting.
The fourth spacer disposed at the rear of the mould, the length of which corresponds to the width of the slab, is inserted between the upper spacer and the lower spacer and can be displaced between these two spacers so as to regulate the length of the slab cast.
In the casting position, the mould is tilted such that its front part lowers in the direction of the casting ladle, so as to form the connection between the filling spout and the exit orifice of the casting ladle.
The closing of the cavity of the mould is provided by applying a transverse clamping force on each of the lateral walls which comes into contact with the assembly of the spacers which are positioned in advance by means of suitable support or suspension devices.
The lateral walls consist of a metallic support on which the graphite blocks forming the inner moulding face of the wall are fixed.
The transverse forces being applied between the two lateral walls, at each of their points, during filling of the mould and during solidification of the metal, are essentially variable so that it is necessary to have available clamping means which have a certain flexibility and permit a certain adaptation of the forces applied in the different zones of the lateral walls. The walls must, in particular, absorb the differences in pressure of the metal and the differential expansions of the various zones of the spacers.
Moreover, clamping the lateral walls against the spacers must be performed efficiently throughout the casting operation in order to avoid leakages of metal and in order to ensure perfect product quality.
The mould must also comprise means for displacement of the lateral walls in the transverse direction, in one direction or in the other, so as to ensure the closing or the opening of the mould.
It has been proposed to connect the lateral walls, on the side of their outer face consisting of the support for the graphite blocks, to extremely rigid longitudinal structure elements consisting of beams whose length is greater than the length of the lateral walls of the mould. The lateral walls are connected to the beams by means of joining means which have a certain elasticity and consist, for example, of rods bearing on springs disposed at several points on the surface of the lateral walls. These elastic joining means make it possible to absorb the differences between the transverse stresses undergone by the walls and the differential expansions.
The clamping of the lateral walls is ensured by devices such as screw jacks inserted between the opposite ends of the beams located on either side of the longitudinal ends of the lateral walls
The beams placed parallel to one another and on either side of the lateral walls, in the transverse direction form, with the screw jacks, a clamping frame which transmits transverse clamping forces to the lateral walls by means of the elastic joining devices.
The beams, whose length must be substantially greater than the length of the walls of the mould, are very long (for example, 14 meters) and the points for application of the stresses of the screw jacks are located at their ends so that these beams undergo an extremely high bending moment during clamping.
In order for clamping to be effective, it is therefore necessary for the beams to have very high rigidity and a very large moment of inertia, so these beams form elements of considerable size and mass.
Moreover, in order to open and close the mould, the beams which are mounted so as to move in the transverse direction on the frame of the mould must comprise means permitting their transverse displacement in one direction and in the other in a totally independent manner.
This design thus leads to an extremely heavy, complicated and costly structure.
In order to lighten and simplify the structure of the mould, it has been proposed for example in the LU-A-49077 to apply the transverse clamping forces of the walls directly on the supports of the graphite blocks forming the outer part of the lateral walls and to exert these transverse clamping forces by means of hydraulic jacks which can also provide the transverse displacement of the walls for opening and closing the mould.
However, this solution requires the use of a very large number of jacks disposed along the lower edge of the lateral wall and inserted between the frame of the mould and this wall as well as a very large number of jacks disposed on the upper part of the lateral walls and ensuring the join between these walls.
The assembly of jacks is fed in parallel via a hydraulic circuit, each of the jacks being connected to the circuit by means of a valve.
During clamping and operation of the installation, each of the jacks is, in fact, subjected to a pressure which depends on the transverse stresses on the wall in the zone where it is fixed. These transverse forces depend, in particular, on the expansion of the graphite wall and on the structure of the mould.
The pressure in each of the jacks depends on the load conditions of this jack and cannot be controlled. The individual valves are thus likely to open one after the other, the stresses at the level of the jack or jacks whose valve is open being distributed over the other jacks whose valves open in turn sequentially. This situation gives rise to "burst" opening of the clamping device.
This device for clamping and displacing the lateral walls by means of hydraulic jacks has the drawback of being extremely complicated due to the fact that a large number of jacks is required and that the feed and control of these jacks involves the presence of numerous hydraulic pipes and numerous valves fixed on various parts of the mould. There is also a risk of a leakage of hydraulic fluid and a risk of pollution by these fluids in the casting zone and the complexity of the installation creates problems which are very difficult to solve during maintenance of this installation.
Moreover, as was explained hereinabove, this complexity is not offset by the operation of the installation being both very reliable and very uniform, in particular on opening.
Moreover, in such an installation, the mechanisms for displacing and clamping the walls consist of the same elements, which somewhat simplifies the opening and closing operations of the mould.
The mould also has a lighter structure than when extremely rigid beams connected at their ends via screw jacks are used for clamping. However, it must be noted that, when only two clamping devices are used, good isostatic clamping is obtained in a satisfactory and simple manner. This condition is not complied with when a very large number of clamping jacks is used.
Moreover, although the overall structure of the mould is considerably lightened, in the case of clamping via the hydraulic jacks being applied directly to the lateral walls the support of the graphite blocks of these walls must be somewhat reinforced compared with the corresponding support of the walls which are fixed elastically on rigid beams.